Gap-Filling Technology

MA-X Complements Side Scan Sonar Imagery

By Dean Fleury

t has been some time Isince something truly unique has come out in sonar imaging technology. That changed this year when Klein Marine Systems introduced MA-X at the Ocean Business Technolo- gy Exhibition in Southamp- ton, U.K., in April. Typical side scan image. MA-X stands for Multi-Angle X-pattern, which describes the approach used to generate images tegrate with an overall system. Thus, there is a need for a with side scan-like characteristics that fills the gap seen system that collects data in the nadir area and creates im- on the screen when using traditional side scan sonars. ages like a side scan sonar system. This means equivalent In a typical side scan image, one can see the black shadows, resolution and tonal quality. To achieve this, stripe down the middle that represents the lack of cov- the system has to ensonify the sea bottom with a grazing erage in the area below the towfish. The lack of cover- angle to create shadows. The frequency and sample rate age under the towfish occurs because the beam patterns of the system have to be carefully chosen to achieve the of the two side scan transducers are oriented in such a needed range and resolution. Finally, the image process- way as to optimize range performance without creating ing has to result in an image similar to and aligned with interference from one to the other. This stripe has been the side scan image. referred to in the industry as the nadir region, water col- umn and even skunk stripe. What it represents is lost Method time and money as the surveying vehicle has to double The simplest description of MA-X is that it is a recon- back and image that area, which is only a fraction of figured side scan sonar, an angle-look sonar, if you will. the system’s imaging swath—leading to the collection of Think of an angle-look sonar embedded in the nose of redundant data. a typical AUV. There are two transducers, port and star- Alternative technologies such as multibeam echo- board, each mounted at an angle relative to the sensor sounders cover the nadir, but their downward-looking platform in such a way that each narrow fan beam is aspect angle results in images that contain no shadows— oriented to look down and forward at an angle from the and it is the shadows in the side scan images that pro- platform. vide the visual cues to both humans and interpretation The starboard transducer puts out a beam that enson- software, such as automated target recognition, to assess ifies the sea bottom starting on the starboard side of the the size of objects and the topography of the terrain. Ad- platform extending diagonally forward to the port side of ditionally, alternative technologies have shortcomings the platform track. The port transducer performs the mir- such as range limitations and differing data formats that ror image. The two fan beams overlap, resulting in an “X” make automated target recognition more difficult to in- pattern on the seafloor. The port and starboard image are

Reprinted from Sea Technology magazine. For more information about the magazine, visit www.sea-technology.com System The MA-X technology is now part of Klein’s MA-X View 600 towfish. The desired functionality of the overall product—high-resolution images from both the side scan sonar and gap-filler sonar—led to the selection of complementary frequencies for the two sensors. The side scan sonar utilizes a focused 600-kHz transmit array to provide excellent imagery out to 50 m. The typical flight altitude of 15 percent of range coupled with the geometry of the MA-X array results in an imaging range of 20 to 40 m, which is a perfect match for Klein’s wideband 850-kHz technology. Note that the side scan image is displayed in slant range, which clearly shows the nadir region, and the MA-X image is in ground range. The imaging is the result of improvements Klein has made throughout the hardware and processing of its product lines, referred to as Klein Blue Technology. Klein Blue represents innovations in acoustics, signal condi- tioning and processing design that produces unmatched image quality and range performance. These innovations include transducers with beam patterns that are nearly indistin- guishable from the theoretical; techniques such as variable transmit power for improved short- range performance; and motion tolerance for consistent imaging in high sea states that impact towfish stability. MA-X transducers can be mounted anywhere that supports the correct mounting angle for the transducers where they have a clear view of the sea bottom ahead. The prototype MA-X View 600 towfish had the transducers mounted to tail fins below the tow body. (Top) MA-X View 600 images. (Middle) The difference in The MA-X View 600 product moved to spring-loaded survey planning for a typical area survey with and without retractable transducers located on the sides of the tow MA-X. (Bottom) Post-processed combined image. The area body, where the MA-X transducers can quickly be moved where the “stitching” occurred is around the 10-m range. into position prior to towfish deployment and snap closed However, this view cuts off a portion of the image from should they impact an undersea object or the side of the each sonar that provides a multi-aspect view of objects in towing vessel during recovery. the area where the two sonars overlap. The combination of the side scan image and the MA-X image provides uninterrupted coverage of the sea bottom, with a varying amount of overlap depending on al- then combined to generate an image of the area normally titude, as a survey vessel passes over a target area. This seen as the gap. reduces the required overlap between survey lines to just The shadows associated with objects on the seafloor the variation associated with ship navigation and results are different than a typical side scan image, where the in a dramatic increase in area coverage rate relative to shadows are perpendicular to the path of the platform. using a side scan sonar alone, conservatively estimated In the MA-X system, the shadows are forward and at an at 40 percent. With traditional side scan sonar alone, the angle to the path of the platform. second swath must overlap the first to cover the nadir gap in the first pass, plus an overlap to account for naviga- littered with a variety of targets. Both the side scan sonar tion variation. The third pass needs overlap for navigation and the gap-filler sonar generated crisp images of targets variation. The process then repeats. ranging from mine-like objects to large sunken structures. When MA-X supplements the side scan sonar, the second pass, and subsequent passes, need only overlap the Conclusion preceding pass by the navigation variation. The MA-X technology is of great interest to users There is no application hungrier for improved efficien- such as the mine countermeasures community, since cy than missions performed by AUVs. AUV customers the imagery has the same interpretation characteristics are clamoring for longer mission times. AUV vendors are as side scan images—grazing angle and high-contrast scrambling to fit larger and larger battery configurations shadowing. Because of this, the data lend themselves into their vehicles. to be processed by automatic target recognition and What if the mission could simply be performed 40 computer-aided detection/computer-aided classification percent faster? software as just another set of “side scan” channels. The In a towed configuration, the increased efficiency increase in area coverage rate makes the technology an with MA-X results in shorter survey times and financial attractive upgrade to the professional survey market. savings associated with fewer labor hours, fuel costs, and The excitement instilled in the sonar community is wear and tear on the towing vehicle. palpable. Initial feedback has ranged from a competitor The options for mounting MA-X in various places on saying, “Why didn’t I think of that?” to a potential cus- the AUV—nose, retractable arrays, tail fins—provides tomer standing up in the middle of a 50-person briefing AUV vendors with options for meeting the increased de- and stating quite loudly, “That is [expletive] amazing!” sire for modularity and rapid reconfiguration of the vehi- Even Marty Klein, “the father of side scan sonar,” was cles for various mission types. awestruck when first exposed to the MA-X images. With its small size, low power consumption and sim- The future looks bright for this new patent-pending ple integration, MA-X can be used in a standalone con- technology. MA-X is a simple and elegant solution that figuration, or it can be combined with a side scan sonar fills the gap without compromising image quality. It does to cover a much larger area in a single swath. The retract- so with a small, low-power system that creates unprece- able-array configuration even provides for the possibility dented imagery capable of being used by a vast array of of using a MA-X system as a high-frequency side scan so- end-users, with no training required. ST nar when the transducers are in their retracted position. Retrofit options allow for deployment of MA-X with side scan sonars or even synthetic aperture sonars from other manufacturers. The first AUV MA-X systems will ship in early 2020. Dean Fleury is the director of engineering at Current consumers of Klein side scan data will find easy Klein Marine Systems. compatibility with MA-X data, which can be delivered in standard .sdf and .xtf formats. Third-party software vendors are already working with MA-X View 600 data. Ini- tial passes at mosaic images have already been submitted for evaluation. A joint exercise was held in Sweden at the end of September where the MA-X View 600 surveyed a lake

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